The invention relates to an X-ray image intensifier tube, comprising an entrance screen which includes a layer of luminescent material, a photocathode and a separating layer which is provided therebetween, and also comprising an electron-optical system images an electron beam, released from the photocathode onto an exit screen.
An X-ray image intensifier tube of this kind is known from GB No. 1,507,370. Therein it is noted that a separating layer must form a chemical barrier between the luminescent layer and the photocathode, which barrier must prevent mutual contamination and must not constitute a substantial barrier for the luminescent layer; moreover, the resolution of the screen may not be substantially reduced thereby.
The solution disclosed therein, utilizing a multiple layer, has several drawbacks. First of all, as appears from the information given, the layer must be thin in order to achieve adequate transmission for the luminescent light. In this context thin is to be understood to mean a thickness which does not exceed the wavelength of the luminescent light. As a result, the function of chemical barrier is liable to be affected and severe requirements must be imposed as regards the deposition technique for the layer which, due to its multiple nature, is already comparatively complex for the known separating layers.
As is stated in U.S. Pat. No. 3,838,273, it is also desirable that the photocathode has a comparatively low electrical resistance in conjunction with a separating layer. In high-resolution X-ray image intensifier tubes, as described in U.S. Pat. No. 3,838,763, this requirement is substantially more severe, because the electrical conductivity of the luminescent layer is liable to be reduced by the columnar structure of the layer, very desirable for reasons of resolution, and the homogeneity of the separating layer and the photocathode may be disturbed thereby. Layers having an adequate electrical conductivity often fail to saitsfy the other requirements, for example in that they must be constructed so as to be too thick for adequate optical transmission.